Low density microcellular carbon or catalytically impregnated carbon foams and process for their prepartion

ABSTRACT

Machinable and structurally stable, low density microcellular carbon, and catalytically impregnated carbon, foams, and process for their preparation, are provided. Pulverized sodium chloride is classified to improve particle size uniformity, and the classified particles may be further mixed with a catalyst material. The particles are cold pressed into a compact having internal pores, and then sintered. The sintered compact is immersed and then submerged in a phenolic polymer solution to uniformly fill the pores of the compact with phenolic polymer. The compact is then heated to pyrolyze the phenolic polymer into carbon in the form of a foam. Then the sodium chloride of the compact is leached away with water, and the remaining product is freeze dried to provide the carbon, or catalytically impregnated carbon, foam.

The U.S. Government has rights in this invention pursuant to ContractNo. w-7405-ENG-48 between the U.S. Department Of Energy and theUniversity Of California for the operation of the Lawrence LivermoreNational Laboratory.

BACKGROUND OF THE INVENTION

The invention described herein relates generally to carbon foams, andespecially to processes for preparing low density microcellular carbon,and catalytically impregnated carbon, foams.

There are many, both presently existing and potential, beneficial usesfor carbon foams. For example, carbon foams have been used as parts forinertial confinement fusion targets, as absorbers of toxic and hazardousgases, and as structural parts requiring unique properties related toX-ray opacity. Very possibly, in applications where large and highlyreactive surface areas of catalysts such as platinum, palladium andnickel must be exposed to unsaturated hydrocarbon gases in theircatalytic decomposition, it appears that it would be highly beneficialto provide low density, microcellular foams comprised of catalyticallyimpregnated carbon. In these and similar applications, it would clearlybe very advantageous to make carbon foams having both a cell size and adensity that were, at the same time, independently controllable. Itwould be especially advantageous to fabricate carbon foamssimultaneously having a low density and a very small cell size. It wouldbe beneficial if the small cell size were very uniform and could betailored to be within the 5 to 30 micron range. Additionally, it wouldbe highly desirable if these tailored carbon foams were free of impurityand structurally stable so that they could be easily fabricated intoparts of various size and shape. Unfortunately, the presently knownmethods for preparing carbon foams are inadequate for producing thesetailored foams.

One prior art method for preparing carbon foam is described by Benton etal in Carbon 10, pages 185 to 190 (1972). In this method, hollowphenolic or carbon microspheres are mixed with a binder materialconsisting of liquid furfuryl alcohol, maleic anhydride, powdered pitch,and acetone. The moist mixture is cured under conditions of elevatedtemperature and pressure. Then the cured mixture is carbonized at hightemperature in an argon-purged furnance. This carbonizing, or cokingoperation produces a significant shrinkage of the resultant carbon foam,which possesses a high compressive strength and a relatively lowdensity. However, the cell size of the carbon foam made by this methodtends to be relatively large, and cell size and density cannot besimultaneously and independently controlled to provide carbon foams oflow density and small cell size.

Processes for producing reticulated, or weblike, polymeric foams byremoving the cell membranes from conventional polymeric foams aredescribed by Geen in the "Encyclopedia of Polymer Science andTechnology", Volume 12, pages 102 to 104, Interscience Publishers(1970). Polyester-derived polyurethane foams may be reticulated byalkaline hydrolysis, which preferentially etches away the foammembranes, leaving an open skeletal structure. In another method, calledexplosion reticulation, the air within a foam is removed and replacedwith an explodable gas mixture. Ignition of the mixture results in acontrolled explosion which removes the thinner membranes. Reticulatedcarbon foams can be produced by the pyrolysis of polyurethane foams thathave been reticulated by either of these methods. Unfortunately, theresulting reticulated carbon foams cannot be fabricated tosimultaneously meet the aforementioned desirable cell size and densityspecifications.

Thus, the problem remains of readily preparing machinable andstructurally stable, tailored low density and microcellular carbon, andcatalytically impregnated carbon, foams.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide machinable andstructurally stable, tailored low density and microcellular carbon, andcatalytically impregnated carbon, foams, and process for theirpreparation.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, machinable and structurally stable, tailored low density andmicrocellular carbon foam may be produced by the process of pressing aquantity of pulverized sodium chloride particles into a sodium chloridecompact, having internal pores, and then sintering the compact. Thesintered compact is then partially immersed in a solution comprised of aphenol-formaldehyde A-stage polymeric phenolic resin of either theresole type or the novolac type dissolved in a solvent selected from thegroup consisting of tetrahydrofuran, acetone, methyl ethyl keytone, andethanol, so that capillary action forces the solution to completely fillthe internal pores of the sintered compact. When the phenolic resin isof the novolac type, it is preferred that the solution additionallycontain a crosslinking agent selected from the group consisting ofhexamethylene tetra-amine and para-formaldehyde. Following immersion,the compact is then submerged in the solution until a uniformconcentration of phenolic resin diffuses throughout the compact. As thenext step, the sintered sodium chloride compact is heated until thephenolic resin pyrolyzes essentially without shrinkage into a carbonfoam whose cell size is determined by the particle size distribution ofthe pulverized sodium chloride particles that comprise the sinteredcompact. After cooling, essentially all the sodium chloride of thesintered compact is leached away with water from the entrained carbonfoam. When the water-wet, carbon foam is freeze dried, the desiredmachinable and structurally stable carbon foam remains.

As an additional and preliminary step in the above described process, itis frequently preferred to classify the pulverized sodium chlorideparticles to remove both fines and large particles. This is accomplishedby passing the pulverized particles through an air classificationsystem. The classified pulverized sodium chloride particles are thenpressed into a compact. when this compact is processed as describedabove, the resulting product is a machinable and structurally stable,low density microcellular carbon foam.

In a further aspect of the present invention, in accordance with itsobjects and purposes, catalytically impregnated carbon foam, that ismachinable and structurally stable, may be prepared by first, beforepressing them into a compact, mixing the pulverized sodium chlorideparticles with a sub-micron size powder comprised of a catalyst materialselected from the group consisting of platinum, palladium, and nickel.These pulverized sodium chloride and catalyst material particles arethen pressed into a compact and processed as described above to producea catalytically impregnated carbon foam that is machinable andstructurally stable.

In yet another aspect of the present invention, if the pulverized sodiumchloride particles are first classified to remove fines and largeparticles, as discussed above, prior to being mixed with catalystmaterial powder, the method of this invention can provide a low densitymicrocellular carbon foam that is catalytically impregnated as well asbeing machinable and structurally stable.

The benefits and advantages of the present invention, as embodied andbroadly described herein, include the provision of low densitymicrocellular carbon foams that are machinable and structurally stable.Further, this invention provides such carbon foams that additionally arecatalytically impregnated.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferred embodimentof the invention. The invention requires, as a starting material,pulverized sodium chloride particles. This is a very common and readilyavailable material. For example, the food grade salt product namedEF325, where the EF stands for Extra Fine, supplied by the MortonThiokol Corporation, Ventron Division, of Danvers, Mass., has been foundto be competent in the performance of this invention. This particularsalt is pulverized to -325 mesh, which means that the sodium chlorideparticles will all pass through a 44 micron sieve. Any similar saltproduct will be at least adeqate in carrying out the processes hereindescribed. For many applications the as-received pulverized sodiumchloride does not require further classification. However, in otherapplications it is required that the as-received pulverized sodiumchloride be classified to remove both fines and large particles. Thisclassification may be beneficially performed by any commerciallyavailable air classification system, such as the ACUCUT Model A-12 airclassification system that is supplied by the Donaldson Company ofMinneapolis, Minn. Air classification systems are commonly and generallyavailable articles of laboratory equipment, and are very well known inthe prior art. As a typical example, when one quantity of Morton EF325salt was classified by an ACUCUT Model A-12 air classification system,the mean particle size, on a weight basis, of the pulverized sodiumchloride typically increased from about 15 to 17 microns, however, theparticle size distribution became considerably narrower. It has beendiscovered that it is often preferable if the processes of thisinvention are carried out with a starting material of sodium chlorideparticles that have been classified, by passage through an airclassification system, to have a mean particle size in the approximaterange from 10 to 20 microns on a weight basis. However, in othersituations it may be beneficial to otherwise tailor the sizedistribution of the pulverized sodium chloride particles, as desired,for example by the process of jet milling.

The next step in the process of this invention is to cold press aquantity of the pulverized sodium chloride particles into a sodiumchloride compact having internal pores. It is preferable that the coldpressing be carried out at a pressure in the approximate range from 2000to 5000 pounds per square inch, for a time of approximately 1 to 5minutes. This results in the sodium chloride compact having a density inthe approximate range from 60 to 65 percent of the theoretical densityof sodium chloride, which is 2.165 gm/cm³. Compacts pressed to a higherdensity than the range indicated tend to have closed voids, which leadsto a discontinuous and thus undesired structure in the resultant carbonfoam as well as limiting the density range thereof. This cold pressingoperation may preferably be performed with any standard laboratorypress, such as the Model C supplied by Fred S. Carver, Inc., asubsidiary of Sterling, Inc. of Menomonee Falls, Wis. Alternatively,this step may be performed by the well-known technique of cold isostaticpressing. In cold pressing sodium chloride particles into compactshaving the shape of bars, it has been observed that plating the innersurfaces of the mold with a nickel-phosphorus coating to inhibitcorrosion, and providing the inner side walls of the mold with a onedegree draft to assist in compact ejection, are advantageous. As apractical matter, it has been observed that it is sometimes advantageousto perform this pressing operation under conditions of reduced humidity.

Following the cold pressing operation, the resulting sodium chloridecompact is sintered. Sintering is performed by heating the compact in alaboratory furnace at the approximate rate of 1 degree centigrade perminute to the temperature of approximately 650 to 725 degreescentigrade, maintaining that temperature for approximately 2 to 12hours, and then shutting off the power to the laboratory furnace andallowing the compact to slowly cool. In the practice of this inventionthis sintering step may be performed by any appropriate laboratoryfurnance, such as the Model Number 11893, three-zone tube furnancesupplied by the Marshall Company of Scotts Valley, Calif. It is alsoconvenient to monitor the sintering operation with one K-type,chromel-alumel, thermocouple per hot zone, and to control the processwith a multichannel programmer/controller such as the MicriconProgrammer/Controller Model Number 823 supplied by Research,Incorporated of Eden Prairie, Minn. However, the sintering may bemonitored by any other appropriate procedure. This sintering operationresults in a noticeable strengthening of the cold pressed sodiumchloride compact. Although particle necking or compact densification arenot observed to significantly occur, the sintering process results in asubstantial reduction in internal surface area, measured in squaremeters per gram, of the sintered compact as compared to the surface areaof classified pulverized sodium chloride particles. It should be pointedout that in some, relatively few, situations the sintering step may beomitted from this procedure by going directly to the following step, butthis is not generally preferred.

After the sodium chloride compact has been sintered, it is nextpartially immersed in a solution comprised of a phenolic polymerdissolved in a solvent selected from the group consisting oftetrahydrofuran, acetone, methyl ethyl keytone and ethanol, so thatcapillary action forces the solution to completely fill the internalpores of the sintered compact without the entrapment of air bubbles. Thesolution should be prepared at the concentration necessary to achievethe desired final carbon foam density. More specifically, the phenolicpolymer may be any phenol-formaldehyde A-stage polymeric phenolic resinof either the resole type or the novolac type. If the resin is of thenovolac type it is preferable, but not absolutely necessary, that thesolution also contain a crosslinking agent selected from the groupconsisting of hexamethylene tetra-amine and para-formaldehyde. If theresin is of the resole type it is self-condensing and thus does notrequire a crosslinking agent. Phenolic polymers, includingphenol-formaldehyde A-stage polymeric phenolic resins, are verywell-known and are described in "Textbook of Polymer Science" by F. W.Billmeyer, Jr., Interscience Publishers (1962), particularly at pages463 to 469, which textbook is incorporated by reference herein. Afterthe internal pores of the sintered compact are completely filled withthe solution, the sintered compact is fully submerged in the solutionuntil a uniform concentration of the phenolic polymer diffusesthroughout the sintered compact. For optimal results, this stepfrequently requires a period of about 24 hours, or even longer. However,if an absolutely uniform concentration of the phenolic polymer is notrequired, a soaking period as short as about 4 hours may be adequate,but this is not preferred. The reason for this submerging step is thatthe phenolic polymer molecules are subject to a filtering orchromatographic effect as they diffuse through the pores of the sinteredcompact, and thus a substantial soaking period is required to give auniform concentration of the phenolic polymer throughout the compact. Itis particularly pointed out that by tailoring the particle sizedistribution of the pulverized sodium chloride particles of the compact,and, at the same time, controlling the concentration of the phenolicpolymer in the solution that is diffused through the compact, both thecell size distribution and the density of the carbon foam of thisinvention may be independently and simultaneously controlled, to providetailored low density and very small cell size, or microcellular, carbonfoams.

Following its removal from the phenolic polymer solution, the sinteredcompact is heated, in a convection oven to approximately 60 degreescentigrade, in air, for approximately 3 hours. This step is effective inremoving the solution solvent from the compact by evaporation. Then thecompact is further heated in a purging atmosphere of a mixture of inertgases selected from the group consisting of argon, nitrogen, andhydrogen to a temperature of approximately 600 to 700 degreescentigrade. The final heating should be performed slowly, over a time ofat least 2 hours, at a rate determined so that thermal shocks do notproduce cracks in the compact. This temperature of approximately 600 to700 degrees centigrade is then maintained for approximately 2 hours,after which it is reduced to the ambient temperature over a time ofapproximately at least 2 hours. Because of this operation, the phenolicpolymer that is diffused throughout the sintered compact pyrolyzesessentially without shrinkage into a carbon foam whose cell size isdetermined by the particle size distribution of the original classified,pulverized sodium chloride particles. That is, crosslinking occurs inthe phenolic polymer and the resulting decomposition gases permeate outof the compact. This heating step may be performed with any appropriatelaboratory furnace, such as the Model Number 5465 6-V-S three-zone tubefurnace, which is supplied by the Lindberg Company, a unit of GeneralSignal Corporation, of Watertown, Wisconsin. The temperature control forthis heating step may be provided by any appropriate means, such asthose described above for the sintering step of this process. Thephenolic polymer generally leaves behind about half of its original massas carbon when slowly pyrolyzed to approximately 600 to 700 degreescentigrade in this operation. The pyrolysis of phenolics has been widelystudied and is described in publications such as "PolymericCarbons-Carbon Fibre, Glass and Char", by Jenkins et al, CambridgeUniversity Press (1976), and "Heat-Resistant Polymers", by Critchley etal, Plenum Press (1983), both of which are incorporated by referenceherein. Thermal expansion mismatches, the 825 degree centigrade meltingpoint of sodium chloride, and the rapid densification of sodium chlorideby sintering above about 750 degrees centigrade, all combine to prohibitthis pyrolyzing step from being performed at any temperature above about725 degrees centigrade.

Subsequent to the pyrolysis of the phenolic polymer to carbon,essentially all the sodium chloride of the sintered sodium chloridecompact is leached away with water. This operation leaves the carbonfoam, in a water-wet condition. This step may be conveniently executedby placing the sintered sodium chloride compact, that contains thecarbon foam, in a dessicator, evacuating the dessicator, admitting waterto the dessicator until the compact is covered with water and water hascompletely diffused throughout the compact without the entrapment of airbubbles, and then removing the vacuum. The water in the dessicatorshould be continuously stirred, and periodically replaced with freshwater until essentially all the sodium chloride is leached away.Alternatively, the wet compact may be subjected to a continuous flow offresh water. As an aside, it is noted that Morton EF325 salt containstricalcium phosphate as an anti-caking agent. This extraneous materialmay be removed at this point with a dilute nitric, or other appropriate,acid wash, with the residual quantity of acid being flushed away withwater. However, it is specifically pointed out that the removal oftricalcium phosphate is not an integral element of the presentinvention. Adequate starting materials provided by other supplies mayrequire other purifying steps of this nature.

The final step in the process is to freeze dry the water-wet carbon foamand thereby provide a machinable and structurally stable, tailored lowdensity and microcellular carbon foam. Typically, the water-wet carbonfoam is frozen at approximately -40 degrees centigrade by being placedupon a tray whose under surface is in contact with an alcohol and dryice bath. Then the carbon foam is placed in a freeze dryer, under roughvacuum, that is initially at -40 degrees centigrade and gradually heatedto approximately -5 degrees centigrade over a time of approximately 4hours, and then held at approximately -5 degrees centigrade forapproximately 4 days. Then the carbon foam is heated to approximately 40degrees centigrade at a rate of approximately 5 degrees centigrade perhour. The carbon foam is then held at this temperature for approximately16 hours. The freeze drying may be performed in conjunction with anyappropriate freeze drying apparatus, such as freeze dryer Model62012343, supplied by the Virtis Company of Gardiner, N.Y. However, theparticular method of freeze drying is not extremely critical. In fact,for higher density carbon foams, one may replace the water in thewater-wet foam with isopropanol by soaking, and then simply heat thefoam at about 60 degrees centigrade in a convection oven until thecarbon foam is dry.

As stated above, all aspects of this invention require, as a startingmaterial, a quantity of pulverized sodium chloride particles, that maybe additionally classified with respect to particle size distribution.When these sodium chloride particles are first mixed with a sub-micronsize powder of a catalyst material selected from the group consisting ofplatinum, palladium, and nickel, the resulting quantity of sodiumchloride and catalyst material particles may be used in the performanceof this invention essentially in the same manner as discussed above.That is, the mixture of particles may be cold pressed into a compacthaving internal pores; the compact may be sintered; the sintered compactmay be immersed and then submerged in a phenolic polymer solution; thephenolic polymer may be pyrolyzed by heating into a carbon foam; thesodium chloride of the particle mixture may be leached away with water;and the water-wet catalytically impregnated carbon foam may be freezedried, all as described above. When this is done, the resulting productis a machinable, structurally stable, tailored low density andmicrocellular carbon foam that is catalytically impregnated. Materialssuch as platinum, palladium, and nickel are referred to herein ascatalyst materials, even though they may actually function catalyticallyonly when they are in very specific and unique configurations, such asin intimate contact with carbon.

The carbon and catalytically impregnated carbon foams of this inventionare machinable on an EMCO FB-2 mill, supplied by Maier and Company ofHallein, Austria, provided with a fine-toothed blade operating at 250 to333 Hertz. However, any generally equivalent mill is appropriate formachining the foams of this invention. Preferably, the foams are held inplace under a partial vacuum during machining. It is also possible tomachine the foams prior to the leaching step of the process of thisinvention, when the foams are still contained within a sodium chloridecompact.

The invention will now be illustrated by the following examples:

EXAMPLE 1

A measure of Morton EF325 pulverized salt was classified by passagethrough an ACUCUT Model A-12 air classification system to provide aclassified quantity of pulverized sodium chloride particles having amean particle size, on a weight basis, of 17 microns. This classifiedquantity of sodium chloride particles was cold pressed, in a mold drivenby a Carver Model C press, at a pressure of 2500 pounds per square inch,for a time of 3 minutes, into a sodium chloride compact with internalpores having the shape of a bar with dimensions 20 by 2.5 by 1centimeters, and having a density of 65 percent of the theoreticaldensity of sodium chloride. This bar was sintered under argon in aMarshall Model 11893 three-zone tube furnace by heating the bar at therate of 1 degree centigrade per minute to 710 degrees centigrade andthen holding it at that temperature for 12 hours. The sintered bar wasthen allowed to cool to ambient temperature. Next, the sintered bar waspartially immersed in a 30 percent weight to volume, i.e., 30 grams to100 millileters, solution of phenolic polymer Number 29-104, aphenol-formaldehyde A-stage polymeric resin of the resole type, suppliedby BTL Specialty Resins Inc., of Niagra Falls, N.Y., dissolved intetrahydrofuran, for 1/2 hour, until the solution completely filled theinternal pores of the compacted and sintered bar, without the entrapmentof air bubbles. Then the bar was fully submerged in the solution for atime of 20 hours, until a uniform concentration of the phenolic polymerhad diffused throughout the bar. Following removal from the solution,the phenolic polymer in the bar was pyrolyzed to carbon by heating thebar in a convection oven to 60 degrees centigrade, in air, for 3 hours,and then further heating the bar to 700 degrees centigrade, slowly overa 12 hour period of time, in a purging atmosphere of argon, in aLindberg Model 5465 6-V-S three-zone tube furnace. The bar wasmaintained at this elevated temperature for 2 hours, and then slowlycooled to ambient room temperature over a period of 16 hours. Then thebar was placed in a dessicator, the dessicator was evacuated, and waterwas admitted to the dessicator until the bar was covered with water andwater had completely diffused throughout the bar without the entrapmentof air bubbles. Then the vacuum was removed. The water was continuouslystirred, and was replaced with fresh water every 8 to 16 hours for a 48hour period. This process leached away essentially all the sodiumchloride from the sintered sodium chloride compacted bar, leaving acarbon foam in a water-wet condition. Finally, the water-wet carbon foamwas frozen at -40 degrees centigrade by being placed upon a tray whoseunder surface was in contact with an alcohol and dry ice bath, and thenplaced in a Virtis Model 62012343 freeze dryer at -40 degrees centigradeand under a rough vacuum. The carbon foam was then gradually heated to-5 degrees centigrade in 4 hours, held at -5 degrees centigrade for 4days, and then heated to 40 degrees centigrade at a rate of 5 degreescentigrade per hour, at which temperature it was then held for 16 hours.Upon removal from the freeze dryer, a machinable and structurally stablecarbon foam having a density of 40 mg/cc, and a median cell size of 15microns, was provided.

EXAMPLE 2

A measure of Morton EF325 pulverized salt was classified by passagethrough an ACUCUT Model A-12 air classification system to provide aclassified quantity of pulverized sodium chloride particles having amean particle size, on a weight basis, of 17 microns. This quantity ofparticles was intimately mixed with sub-micron size platinum powersupplied by Englehard, Corp., of Iselin, N.J., the platinum powercomprising 2 weight percent of the resulting mixture. The sodiumchloride and platinum mixture of particles was cold pressed, in a molddriven by a Carver Model C press, at a pressure of 2,500 pounds persquare inch, for a time of 3 minutes, into a platinum impregnated sodiumchloride compact with internal pores having the shape of a bar withdimensions 20 by 2.5 by 1 centimeters. The bar was sintered under argonin a Marshall Model 11893 three-zone tube furnace by heating the bar atthe rate of 1 degree centigrade per minute to 710 degrees centigrade andthen holding it at that temperature for 12 hours. The sintered bar wasthen allowed to cool to ambient temperature. Next, the sintered bar waspartially immersed in a 35 percent weight to volume solution of phenolicpolymer Number 29-104, a phenol-formaldehyde A-stage polymeric resin ofthe resole type, supplied by BTL Specialty Resins, Inc., of NiagraFalls, N.Y., dissolved in acetone for 1/2 hour, until the solutioncompletely filled the internal pores of the compacted and sintered bar,without the entrapment of air bubbles. Then the bar was fully submergedin the solution for a time of 24 hours, until a uniform concentration ofthe phenolic polymer had diffused throughout the bar. Following removalfrom the solution, the phenolic polymer in the bar was pyrolyzed tocarbon by heating the bar in a convection oven to 60 degrees centigrade,in air, for 3 hours, and then further heating the bar to 700 degreescentigrade, slowly over a 12 hour period of time, in a purgingatmosphere of argon, in a Lindberg Model 5465 6-V-S three-zone tubefurnace. The bar was maintained at this elevated temperature for 2hours, and then slowly cooled to ambient room temperature over a periodof 16 hours. Then the bar was placed in a dessicator, the dessicator wasevacuated, and water was admitted to the dessicator until the bar wascovered with water and water had completely diffused throughout the barwithout the entrapment of air bubbles. Then the vacuum was removed. Thewater was continuously stirred, and was replaced with fresh water every8 to 16 hours for a 48 hour period. This process leached awayessentially all the sodium chloride from the sintered and platinumimpregnated sodium chloride compacted bar, leaving a platinumimpregnated carbon foam in a water-wet condition. Finally, thewater-wet, platinum impregnated carbon foam was frozen at -40 degreescentigrade by being placed upon a tray whose under surface was incontact with an alcohol and dry ice bath, and then was placed in aVirtis Model 62012343 freeze dryer at -40 degrees centigrade and under arough vacuum. The platinum impregnated carbon foam was then graduallyheated to -5 degrees centigrade in 4 hours, held at -5 degreescentigrade for 4 days, and then heated to 40 degrees centigrade at arate of 5 degrees centigrade per hour, at which temperature it was thenheld for 16 hours. Upon removal from the freeze dryer, a machinable,structurally stable, platinum impregnated carbon foam having a densityof 80 mg/cc, and a median cell size of 15 microns, was provided.

It is thus appreciated that in accordance with the invention as hereindescribed, both machinable and structurally stable, tailored low densityand microcellular carbon, and catalytically impregnated carbon, foams,and process for their ready preparation, are provided.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. For example, instead of using pulverizedsodium chloride particles, potassium chloride particles or very smallglass spheres or particles may be pressed into compacts suitable for theperformance of variations of the present invention. Of course, whenusing glass particles, the leaching step must be performed withhydrofluoric acid instead of water. Additionally, immersing solutionscomprised of epoxy resin or furfuryl alcohol monomer may be pyrolyzedinto carbon foams in accordance with the basic teaching of thisinvention. Also, instead of simply mixing catalyst material powders withpulverized sodium chloride particles, the catalyst material may bevacuum deposited onto pulverized sodium chloride particles by techniquesthat are substantially disclosed by Land in U.S. Pat. No. 3,295,972issued Jan. 3, 1967. Additionally, catalyst material may be diffusedinto the carbon foams of this invention by allowing the foams, whenwater-wet, to soak in aqueous solutions containing the catalystmaterials, such as dilute chloroplatinic acid, for appropriate periodsof time. when performing this technique, following freeze drying, thecarbon foams must be reheated in a purging atmosphere to decompose thecompound that contains the catalyst material.

The embodiment was chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

We claim:
 1. A process for producing a machinable and structurallystable, low density microcellular carbon foam having a density of about40 mg/cc and a median cell size of about 15 microns, the processcomprising the steps of:classifying a measure of pulverized sodiumchloride particles, by passing the measure through an air classificationsystem, to remove both fines and large particles, to thereby provide aclassified quantity of pulverized sodium chloride particles having amean particle size in the approximate range from 10 to 20 microns; coldpressing the classified quantity of pulverized sodium chlorideparticles, at a pressure in the approximate range from 2000 to 5000pounds per square inch, and for a time of approximately 1 to 5 minutes,into a sodium chloride compact having a density in the approximate rangeof 60 to 65 percent of the theoretical density of sodium chloride, andwith the compact having internal pores; sintering and therebystrengthening the sodium chloride compact, by heating the compact at theapproximate rate of 1 degree centigrade per minute to a temperature inthe approximate range from 650 to 725 degrees centigrade, then bymaintaining the compact at that temperature for approximately 2 to 12hours, and then by cooling the compact to ambient temperature; preparinga solution comprised of a resole or novolac phenol-formaldehyde A-stagepolymeric phenolic resin dissolved in a solvent selected from the groupconsisting of tetrahydrofuran, acetone, methyl ethyl keytone, andethanol; partially immersing the sintered sodium chloride compact in thesolution so that capillary action forces the solution to completely fillthe internal pores of the sintered compact without the entrapment of airbubbles, and then submerging the sintered compact in the solution untila uniform concentration of the phenolic resin has diffused throughoutthe sintered compact; heating the sintered sodium chloride compact,through which the phenolic resin has uniformly diffused, toapproximately 60 degrees centigrade in air for approximately 3 hours,and then further heating the compact in a purging atmosphere comprisedof a mixture of inert gases selected from the group consisting of argon,nitrogen, and hydrogen, to a temperature in the approximate range from600 to 700 degrees centigrade, then maintaining the sintered commpact inthe purging atmosphere and at that temperature for approximately 2hours, so that the phenolic resin that is diffused throughout thesintered compact pyrolyzes essentially without shrinkage into a carbonfoam whose cell size is determined by the particle size distribution ofthe classified quantity of pulverized sodium chloride particles;leaching away with water essentially all the sodium chloride from thesintered sodium chloride compact, so that the carbon foam in a water-wetcondition remains; and freeze drying the water-wet carbon foam tothereby provide said machinable and structurally stable, low densitymicrocellular carbon foam.
 2. A process as recited in claim 1, saidresin is a novolac phenolic resin and, in which the solution furthercomprises a crosslinking agent selected from the group consisting ofhexamethylene tetra-amine and para-formaldehyde.